1. Field of the Invention
The present invention relates to a collet chuck for holding a tool or workpiece on the spindle of a machine tool. More particularly, the present invention relates to a collet chuck having a dead length work stop.
2. Description of the Prior Art
Collets are sleeves or collars used in various machine tools such as lathes for clamping or gripping workpieces or tools. Collets have conical surfaces or cams, which, when the collet is pulled back, interact with corresponding, opposing surfaces or cams on a mounting fixture or on the spindle. This interaction causes the collet to contract or expand to grasp or release a work piece or tool, depending on the direction of movement. Collet chucks have commonly been used in machine tools in place of other types of chucks. Collet chucks are typically more accurate and have a greater gripping characteristic than a typical jaw chuck, for example. An advantage of collets is that they continue to grasp the workpiece or tool even at high rotational speeds when jaw chucks would have a tendency to loosen their grip due to centrifugal force.
One problem encountered with collet chucks is that slight variations in the diameter of the workpiece or stock could cause the collet to position the workpiece differently. When and where a collet will grasp a work piece depends on the difference in diameter between the open collet and the diameter of the workpiece. Precise workpiece diameter is therefore required if the workpiece is to be positioned precisely and consistently in machining operations such as facing, side finishing or cutting to precise lengths.
Dead length stops of different kinds have been proposed in the art to solve the problem of workpiece positioning with machine collets. One such work stop is disclosed in U.S. Pat. No. 4,505,487 to Davignon et al. The work stop described in this patent comprises a number of fins radiating from the body of the stop in between the slots or segments of the collet. The fins serve to engage the inside of the spindle and therefore fix the stop in a given position. This structure has the disadvantage that it works inside of the spindle to fix the stop. Such a structure, while it may serve to provide a dead stop for a workpiece, is not capable of being incorporated into a collet chuck that can easily replace a normal chuck on a lathe. In other words, it can only be mounted inside the spindle in a lathe or other machine tool that is dedicated to the use of collet chucks.
A collet assembly capable of being mounted in front of the spindle in the manner of a jaw chuck is disclosed in U.S. Pat. No. 5,096,213 to Terwilliger et al. The assembly disclosed in this patent comprises a two-part collet having a collet head and a collet body. The collet head has a conical outer surface at its front end that interacts with a corresponding cam, on the collet mount, and an inside surface for gripping the workpiece. A flange portion at the front end of the collet body engages a hook and groove portion of the collet head, so that if the stroke of the collet exceeds a certain amount. As often happens when the chuck is mistakenly closed without a workpiece in place, the flange of the body "pops" out of the groove, possibly resulting in damage to the collet chuck assembly. This assembly has the disadvantage that there is no way of limiting the stroke of the collet before the flange pops out.
A collet chuck having a dead length stop is disclosed in U.S. Pat. No. 4,702,484 to Sandwick. The disclosed collet chuck has an outer collet body capable of being connected to the spindle of a lathe. A slotted collet, having gripping surfaces on the radially inward portion of the front end and cams on the radially outward portion of this end, is disposed inside the outer collet body. The collet closes in a conventional manner when it is pulled back by a tubular chuck actuating element. The cams of the collet then interact with an external flange at the front end of the outer collet body. The dead length stop is formed by a ring portion which is located radially inward of the collet and has a certain clearance from the collet. The ring is threaded around a cup-like stop. Centering pins, which are threaded into holes in the outer collet body, hold the ring and cup stop in place. The pins are located between the slots of the collet and therefore do not interfere with the functioning of the collet. The position of the stop can be adjusted by turning the externally-threaded cup stop relative to the ring until the stop reaches a desired axial position. A second, rod-like work stop is threaded into the cup-like stop at the axis of rotation and can be positioned independently of the cup stop.
There are, however, certain disadvantages of the above-described collet chuck. Firstly, there is nothing within the chuck itself to limit the stroke of the collet, and excessive stroke will result in damage to the collet. Secondly, variations during manufacture of such collet assemblies result in clearances which could cause the stop to migrate during a machining operation and result in slight variations in the length of pieces machined using this type of collet stop. Also, variations in the machining of the chuck itself can not be compensated for because the pins that position the stop threadedly engage the outer collet body.
In view of the above problems with the prior art collet chucks, it is an object of the present invention to provide a collet chuck that can be readily adapted for use with machine tools in which conventional jaw chucks are also used.
It is another object of the present invention to provide a collet chuck in which the length of the workpiece is determined independently of variations in diameter of the workpiece.
It is a further object of the present invention to provide a collet chuck in which the work stop is adjusted independently of the collet.
It is an object of the present invention to provide a collet chuck in which the stroke of the collet can be limited to prevent damage to the collet.
It also is an object of the present invention to provide a collet chuck with a dead length workstop in which variations in the manufacture of the collet and stop do not affect the accuracy of the length of workpieces obtained using the chuck.